Musical Fidelity 750K Supercharger monoblock power amplifier

Musical Fidelity's "Supercharger" concept is simple, which is perhaps why no one had thought of it before: If you love the sound of your low-powered amplifier but your speakers are insensitive, or you just need more loudness, you insert the high-power Supercharger amplifier between your low-powered amp and speakers. The Supercharger loads the small amplifier with an easy-to-drive 50 ohms, and, in theory, has so little sonic signature itself that it passes on the sonic signature of the small amp unchanged, but louder.

Michael Fremer reviewed Musical Fidelity's first Supercharger amplifier, the 550W 550K ($5000/pair), in September 2007. Using the 550K both as a traditional monoblock power amplifier and as a Supercharger to increase the dynamic range of his beloved Music Reference RM200 tube amp, a 1964-vintage Scott 299D integrated amplifier, and some solid-state designs, Mikey was impressed by what he heard. "Using a variety of very different-sounding amplifiers of various power outputs overwhelmingly demonstrated to me that the 550K Supercharger will retain the sonic attributes of your favorite low- or medium-powered amp (50200Wpc), whether tubed or solid-state, while increasing its output by 10dB or more....The result will be dynamic realism and, in most cases, better overall performance. You can have your cake and make it rock, too."

In my own auditioning of the same pair of 550K Superchargers, used as conventional monoblock amplifiers, I was very impressed by their effortless dynamics and iron-fisted control of the loudspeakers' bass. However, I ultimately felt that the 550Ks did have some character, sounding lean compared with my reference Mark Levinson No.33H monoblocks, and with a less liquid midrange. Overall, the 550Ks sounded very similar to Musical Fidelity's flagship dual-mono kW behemoth, which Michael Fremer reviewed in January 2004, but perhaps with less delicacy (footnote 1).

Of Musical Fidelity's high-powered amplifiers, I much preferred the balance of the kW750 (750Wpc into 8 ohms, $10,000). I had first encountered this stereo design when I used it to drive Wilson Sophias 2s, playing some of my high-resolution recordings at a pair of musical evenings promoted by North Carolina dealer Audio Advice in December 2005. The kW750 combined the kW's extraordinary dynamic range and control of the woofers with a warmer lower midrange and sweeter-sounding high frequencies. Mikey found it too mellow compared with his kWs, though that is perhaps a matter of taste. After using it in my system for a while, I seriously considered buying a kW750, but a cooler financial head than mine prevailed. So when MF's Antony Michaelson told me that he was introducing a Supercharger based on the 750K's circuitry, I asked for a pair for review (footnote 2).

The 750K
At first glance, the 750K Supercharger looks identical to the 550K: a black cylinder topped with an aluminum cap, made in Taiwan. It has the same music-sensing turn-on/off circuit and the same three LEDs at its base: red for standby, blue for operation, orange for thermal overload. However, while the new amplifier shares the 550K's 8.5" diameter, it is just over 6" taller, and its aluminum cap has a mesh-covered vent, through which two temperature-controlled fans exhaust hot air. (There are discreet inlet vents at the sides; the fans run briefly when the amplifier is first switched on, then remain off until the heatsink temperature rises above a preset threshold.) There is now a balanced XLR input jack on the rear panel in addition to the 550K's single-ended RCA. The maximum power is specified as 750W into 8 ohms or 1150W into 4 ohms, an increase of 1.75dB compared with the 550K, though the price is 6dB higher: $10,000/pair compared with $5000/pair.

Sound
Psychoacousticians tell us that our aural memories are reliable only in the short term (though that doesn't tie in with the fact that we instantly recognize friends' voices on the phone despite the lack of fidelity). But from the instant I powered up the 750K Superchargers in my system, using them as conventional monoblocks from their balanced inputs, I was immediately reminded of the kW750. A warmish midrange, sweet-toned high frequencies, tight, deep low frequencies, and a voluminous, stable, well-defined soundstageall were exactly what I remembered of the sound of the kW750 in my system, back in the day.

The images of the singers in Cantus's luminous performance of Eric Whitacre's Lux Aurumque, from While You Are Alive, the recording I made with them in summer 2007 (CD, Cantus CTS-1208), were precisely positioned in space; it was very easy to perceive when the tenors turned away from the microphones to add spaciousness to their sound. And the character of the voices was as natural-sounding and as unforced as I expected, with no added hardness in the climaxes of the suite, A Sound Like This, by Edie Hill also featured on the CD. (Male voices singing close harmonies at high levels provide the perfect test signal to reveal shortcomings in amplifiers and loudspeakers.)

The Musical Fidelity's enormous dynamic range and bass control got the best from Live at Merkin Hall, my recording of Stereophile reviewer Bob Reina's jazz group, Attention Screen (CD, Stereophile STPH018-2). I use as few mikes as possible when I record a drum kit: two cardioids overhead as an ORTF pair, a Shure cardioid clipped just above the snare drum's top skin, and an AKG dynamic mike in front of the kick drum's front skin. I time-align the outputs of the two spot mikes with the outputs of the cardioid pair, my goal being to capture both a natural image of the drums and their natural dynamic range. With an empathetic drummer capable of optimally tuning his kiteg, Attention Screen's Mark Flynnalmost no equalization or compression is required in postproduction. And Mark hit the heck out of his Gretsch kit that February night in Merkin Hall. There are some snare-drum shots on "Blizzard Limbs"the three beats at 3:40 that divide the rocking improvisation that begins the piece from the more contemplative second section, for examplethat go from 60 to 0dBFS from one sample to the next. Amplifiers that can't swing as many volts as the 750K will clip those peaks, unless you play the music too quietly. With the Musical Fidelitys, I could play this track at live levels without waveform clipping.

It being the end of Zeptember as I write these words, I had to get the Led out, specifically How the West Was Won (DVD-Audio, Atlantic 83587-9), recorded live at two L.A. concerts in 1972 by Eddie Kramer. Yes, suck-and-blow compression is obvious at times, but this set features great recorded drum sound, with tangible space around and between the drums. Forget "Stairway to Heaven," "Immigrant Song," "Whole Lotta Love"the highlight of this album is the blues "Since I've Been Loving You." Even at ear-bleed levelsis there any other way to listen to Led Zeppelin?the Superchargers allowed me to hear into the layering of the soundstage, with Bonzo's drums behind Jimmy Page's guitar and "Percy" Plant's wailing.

Footnote 1: In May 2008, I used the review samples of the 550K for a rather unusual purpose: driving Revel Performa F30 loudspeakers as a super-high-quality sound-reinforcement system for a series of concerts by male choir Cantus in Minneapolis's Southern Theater. Following the shows, I asked Cantus producer Erick Lichte, with whom I have worked for eight CDs now and whose ears I respect immensely, to give the 550Ks a listen, using them to Supercharge his sweet-sounding Pass Labs Aleph 3. His report appears in this issue's Follow-Up section.

Footnote 2: Readers should note that, at the end of 2003, Musical Fidelity's Antony Michaelson asked me to produce a recording of him performing Mozart's Clarinet Concerto, K.622, which was released on SACD and LP. To avoid any appearance of a conflict of interest, I waived any fee and paid my own expenses to attend the sessions in London's Henry Wood Hall. I also imposed a moratorium of five years on my writing a review of a Musical Fidelity product (other than the usual measurement sections that accompany reviews by other Stereophile writers).